Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes

[David Gibson]

On Sun, Aug 18, 2019 at 10:59:01PM +0200, Aleksandar Markovic wrote:
> 18.08.2019. 10.10, "Richard Henderson" <address@hidden> је
> написао/ла:
> >
> > On 8/16/19 11:59 PM, Aleksandar Markovic wrote:
> > >> From: "Paul A. Clarke" <address@hidden>
> > ...
> > >>   ISA 3.0B has xscvdpspn leaving its result in word 1 of the target
> > > register,
> > >>   and mffprwz expecting its input to come from word 0 of the source
> > > register.
> > >>   This sequence fails with QEMU, as a shift is required between those
> two
> > >>   instructions.  However, the hardware splats the result to both word 0
> > > and
> > >>   word 1 of its output register, so the shift is not necessary.
> > >>   Expect a future revision of the ISA to specify this behavior.
> > >>
> > >
> > > Hmmm... Isn't this a gcc bug (using undocumented hardware feature),
> given
> > > everything you said here?
> >
> > The key here is "expect a future revision of the ISA to specify this
> behavior".
> >
> > It's clearly within IBM's purview to adjust the specification to document
> a
> > previously undocumented hardware feature.
> >
> 
> By no means, yes, the key is in ISA documentation. But, the impression that
> full original commit message conveys is that the main reason for change is
> gcc behavior. If we accepted in general that gcc behavior determines QEMU
> behavior, I am afraid we would be on a very slippery slope - therefore I
> think the commit message (and possible code comment) should, in my opinion,
> mention ISA docs as the central reason for change. Paul, is there any
> tentative release date of the new ISA specification?

It's not really a question of gcc behaviour, it's a question of actual
cpu behaviour versus ISA document.  Which one qemu should follow is
somewhat debatable, but it sounds here like the ISA will be updated to
match the cpu, which weights it heavily in favour of mimicing the
actual cpu.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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